Foal heat is the first estrus (heat cycle) a mare experiences after giving birth, typically occurring between 5 and 11 days postpartum. It represents the mare’s remarkably fast return to reproductive readiness, and it’s a key decision point for breeders: whether to breed on this early cycle or wait for the next one. The choice involves trade-offs between gaining time in the breeding season and accepting somewhat lower conception rates.
When Foal Heat Occurs
Most mares enter foal heat within about a week of foaling, with the window spanning roughly day 5 through day 11 after birth. Ovulation during foal heat averages around day 12 to 13 postpartum, though individual mares vary widely. Some ovulate as early as day 8, others closer to day 17.
This early return to cycling is possible because the equine uterus recovers at a speed unusual among large mammals. Remnants of the placenta and shed cells clear from the uterine lining within about 5 days of foaling. The uterine glands, which shift into a heavy secretory mode during pregnancy, also return to their normal appearance within that same 5-day window. White blood cells that show up during this repair process are a normal part of healing, not necessarily a sign of infection.
Conception Rates and Pregnancy Loss
Breeding on foal heat works, but the numbers are consistently lower than breeding on a later cycle. In one study of Finn horse mares, the conception rate at foal heat was about 46%, compared to a cumulative rate above 94% when breeders used the second or third postpartum cycle. Another study of Paso Fino mares reported 71% of mares becoming pregnant at foal heat, which is respectable but still leaves room for improvement by waiting.
Pregnancy loss is the other concern. Mares bred at foal heat are roughly 1.9 times more likely to lose the pregnancy than mares bred on a subsequent cycle. Across multiple studies, pregnancy loss rates for foal heat breedings ranged from about 7.5% to 16.5%, while later breedings ranged from about 3.8% to 14.3%. The gap isn’t enormous in every study, but it’s consistent enough to factor into planning.
Two factors strongly influence these numbers. First, timing within foal heat matters: mares bred after day 10 postpartum have higher pregnancy rates than those bred earlier, likely because the uterus has had more time to recover. Second, uterine fluid accumulation during foal heat increases the risk of embryonic death. A mare whose uterus still holds excess fluid is a poorer candidate for breeding on this cycle.
What Makes Some Mares Poor Candidates
Not every mare should be bred at foal heat. Mares that had a difficult delivery (dystocia), retained their placenta, or developed a uterine infection afterward are typically excluded. The placenta retention threshold matters more than you might expect: mares that passed their placenta within 3 hours had foal heat pregnancy rates around 50%, while those that took longer than 3 hours dropped below 20%.
Stress also plays a role. Mares exposed to higher stress levels after foaling show greater embryo loss when bred at foal heat compared to mares in calmer environments. This is one of those factors that’s harder to quantify but worth considering, particularly for mares adjusting to new surroundings or dealing with health complications.
How Vets Assess Readiness
Veterinarians use ultrasound to evaluate whether a mare is actually ready for breeding during foal heat. They’re looking at several things: the size of the developing follicle on the ovary, the degree of swelling (edema) in the uterine wall, and whether fluid has accumulated inside the uterus.
In most light-breed mares, ovulation happens when the dominant follicle reaches 40 to 45 mm in diameter, though the range can be surprisingly broad, anywhere from 20 to 70 mm. A small amount of clear fluid in the uterus is normal during estrus and the early postpartum period. A large accumulation, however, signals a problem that needs treatment before breeding can proceed. Beyond the ultrasound, vets also confirm heat signs like a relaxed cervix and a positive response when the mare is teased with a stallion.
Skipping Foal Heat With Hormones
Many breeders choose to skip foal heat entirely and use a hormone called prostaglandin to bring the mare back into heat on a more controlled schedule. The approach works like this: the mare ovulates during foal heat but isn’t bred. About 5 days after that ovulation, the prostaglandin injection causes the newly formed hormone-producing structure on the ovary to break down, ending the cycle early and triggering a new heat.
After treatment, mares typically return to heat within 2 to 5 days, with ovulation following roughly 7 to 10 days after the injection. The exact timeline depends on where the mare’s follicles are in their growth phase. A mare with a large, nearly mature follicle may ovulate as soon as 48 to 72 hours after treatment. One whose follicle is still small or regressing could take up to 15 days. Ultrasound monitoring helps predict which scenario applies.
This “short-cycling” strategy gives the uterus additional recovery time while still keeping the mare on a relatively tight breeding schedule, often resulting in breeding around 3 to 4 weeks postpartum instead of the 5 to 6 weeks it would take to wait for the natural second cycle.
Foal Heat Diarrhea in the Foal
The term “foal heat” also comes up in a completely different context: foal heat diarrhea. This is a bout of loose stool that affects foals between about 5 and 15 days of age, coinciding with the mare’s foal heat (which is how it got its name). Despite the timing, the diarrhea doesn’t appear to be caused by hormonal changes in the mare’s milk.
The actual cause remains uncertain. It seems to reflect the foal’s intestinal tract maturing and its gut bacteria shifting as it begins sampling the outside world. Parasite exposure and coprophagy (foals eating the mare’s manure, which is normal behavior that helps establish gut flora) have both been suggested as contributing factors. The diarrhea is typically self-limiting and resolves on its own, though foals should be monitored for dehydration or signs that something more serious is going on.